The present invention relates generally to refractory metal intermetallic composites and methods for preparing such materials. Some specific embodiments of the invention are directed to core constructions used in casting the materials into useful articles.
Turbines and other types of high-performance equipment are designed to operate in a very demanding environment which usually includes high-temperature exposure, and often includes high stress and high pressure. Turbine components have often been fabricated from a class of materials known as superalloys, which characteristically exhibit desirable chemical and physical properties under the service conditions generally experienced during turbine operation. Superalloys in which the base constituent is an alloy of nickel (Ni), iron (Fe), or cobalt (Co) are of particular interest in such applications, because of their ability to withstand the normally high operating temperatures of the turbine service environment.
More recently, a variety of new compositions have been developed to meet an ever-increasing threshold for high-temperature exposure. Prominent among such materials are the refractory metal intermetallic composites (RMIC's). Examples include various niobium-silicide alloys. (The RMIC materials may also include a variety of other elements, such as titanium, hafnium, aluminum, and chromium). These materials generally have much greater temperature capabilities than the current class of nickel- and cobalt-based superalloys. As an illustration, while many nickel-based superalloys have an operating temperature limit of about 1100° C., many RMIC alloys have an operating temperature in the range of about 1200° C.-1700° C. These temperature capabilities provide tremendous opportunities for future applications of the RMIC alloys (which are usually formed as single crystal and directionally-solidified castings). Moreover, the alloys are considerably lighter than many of the nickel-based superalloys.
Both superalloy and RMIC materials can be cast into useful articles, using a variety of techniques. An example of a popular technique is investment casting, sometimes referred to as the “lost wax process”. Gas turbine engine blades and vanes (airfoils) are usually formed by this type of casting technique.
Turbine engine components such as airfoils usually require a selected structure of interior passageways. In most instances, the passageways function as channels for the flow of cooling air. During operation of the turbine engine, the cooling air maintains the temperature of the component within an acceptable range.
The interior passageways in these components are typically formed by the use of one or more cores. (The cores can be used to form various other holes and cavities as well). In a typical process, a ceramic core is positioned within an investment shell mold. After casting of the part, the core is removed by conventional techniques. As described below, cores can be formed of many materials, e.g., ceramic oxides such as silica, alumina, and yttria (yttrium oxide). A typical core for a turbine engine blade is depicted in U.S. Pat. No. 5,580,837 (Dodds et al).
As practiced in the art, green (unfired) cores are usually formed in desired core configurations by molding or pouring the appropriate ceramic material, with a suitable binder and other additives, into a suitably-shaped core die. After the green core is removed from the die, it is subjected to firing at elevated temperatures (usually above about 1000° C.) in one or more steps, to remove the fugitive binder, and to sinter and strengthen the core. As a result of the removal of the binder and any fillers, the fired ceramic core is porous.
In the case of casting high-performance components, cores for the molds must possess a very specific set of attributes. The core must be dimensionally stable and sufficiently strong to contain and shape the casting. Dimensional accuracy and stability are especially important in the case of many turbine components, e.g., airfoils having intricate internal passageways. Heating of the core at or above the casting temperature is often necessary prior to casting, to provide some temperature-stabilization within the core body. However, this heat treatment can lead to an undesirable amount of shrinkage. If the core were to exhibit shrinkage of greater than about 0.2% in some situations, the required dimensional accuracy and stability are difficult to achieve. In other situations, a greater degree of shrinkage can be tolerated, but “shrinkage reproducibility” may be a critical characteristic.
Core strength is also a very important consideration—especially in the case of casting high-melting materials like the RMIC's. In those cases, the strength of the core after firing must often be very high, e.g., greater than about 500 psi. High casting temperatures also require that the core have excellent refractory characteristics.
In addition to dimensional stability and a certain degree of strength, the core must also have a low “crush strength”, so that the ceramic material of the core will crush before the metal being cast is subjected to tensile stress. (Otherwise, tensile stress could lead to mechanical rupture of the casting during solidification and cooling). Moreover, it is often critical that the microstructure and composition of the core allow for relatively easy removal after casting. Removal of the core is typically accomplished by leaching processes, along with other mechanical removal techniques. The porosity level of the core can be very important for minimizing compressive strength and facilitating core removal.
In many instances, the core must also be chemically inert. As an example, when casting highly reactive materials like the RMIC's, any reaction between the casting metal and certain components in the core can result in serious defects on the interior surfaces of the cast article. Niobium silicide castings are especially susceptible to adverse reaction when brought into contact at elevated temperatures with free silica and alumina from the core. (The same concern is sometimes present when casting nickel-based superalloy materials as well, e.g., when the superalloy contains substantial amounts of yttrium or hafnium).
Many ceramic compositions have been developed to address a variety of requirements for casting cores. As an example, U.S. Pat. No. 4,097,292 (Huseby and Klug) describes core compositions based on various yttrium aluminates, including yttrium aluminum perovskite (YAP), yttrium aluminum garnet (YAG), and monoclinic yttrium aluminate (YAM). The compositions consist of either single phase materials of the aluminates, or two-phase mixtures thereof. As another example, U.S. Pat. No. 5,409,871 (Dodds and Alexander) describes core compositions based on yttrium aluminate and alumina (Al2O3). Moreover, U.S. Pat. No. 6,345,663 (Klug and Giddings) describes casting core precursor materials which include alumina, aluminum metal, a polymerizable binder, and various water/organic solutions. The precursor slurry can also contain various other constituents, such as colloidal silica, hafnium, yttrium aluminate, other rare earth aluminates, magnesium, and zirconium.
It can be very difficult to attain all of the advantageous characteristics for ceramic cores, by way of a single formulation. As an illustration, while certain core materials may exhibit the high strength required for casting, they fail to exhibit the low crush strength required to prevent hot-cracking of the metal during cooling. In other cases, core materials may exhibit the required degree of both strength and stability, but fail to possess the desired “leachability” characteristics, which can be a very serious disadvantage. In still other cases, core materials meet or surpass specifications for all of these properties, but do not exhibit the chemical inertness required for casting materials like the RMIC's.
Thus, there continues to be great interest in designing unique core constructions and core fabrication processes. These innovations should help to satisfy the future demands of efficiently casting high-quality metallic alloys and composites, such as superalloy and RMIC materials.